The invention concerns a magnetic resonance tomography apparatus comprising a gradient system that can generate at least one spatially varying and optionally time-varying magnetic field for at least one-dimensional local encoding of measuring signals in an area of a test sample to be imaged.
A magnetic resonance tomography apparatus of this type is disclosed e.g. in “Magnetic Resonance Imaging”, Ed. MT Vlaardingerbroek, Springer-Verlag.
In NMR tomography, the properties of atomic nuclei, in particular 1H nuclei, are utilized for imaging. The nuclear spins are excited in a test sample, which is located in a strong static magnetic field, by suitable RF (radio frequency) pulses, and the RF response of the atomic nuclei is read-out using suitable receiver coils
Encoding methods are used to associate a measuring signal with a location in the test sample. In particular, magnetic fields with a field strength which monotonically (and usually linearly) varies at least in one direction across the entire test sample are thereby used (“conventional gradients”). The local magnetic field strength determines the local Larmor frequency. The Larmor frequency can be uniquely associated with the location in the test sample for monotonic encoding field distributions. The local resolution and the measuring time depend on the gradient strength (steepness of the magnetic field). Since several conventional gradients have to be used successively for local encoding in more than one dimension, these magnetic fields must moreover be switched very quickly. The interaction with the main field of the magnet generates Lorentz forces.
One disadvantage of this prior art is that the rapidly changing Lorentz forces, that are generated in particular in larger areas of a test sample to be imaged when the monotonically extending conventional gradients rapidly change, produce a substantial mechanical load, and the deformation caused by these forces generates substantial noise of typically 100 dBA or more.
The gradient strength which is required to obtain typical local resolution and measuring time causes enormous differences in the magnetic field at the edge of the area to be imaged. Rapidly changing magnetic field strengths may stimulate a patient's neurons in medical applications, such that the measuring time of an NMR acquisition using conventional gradients is eventually limited by physiological factors (noise, neuronal stimulation).
Neuronal stimulations can be minimized using local gradient systems, in which a strong gradient acts only on a short path in each case, such that the local change of the field dB/dt which is relevant for the stimulation is small. However, such local gradient systems produce strong mechanical interaction with the main field causing very strong vibration and hence increased noise.
One further conventional method suggests a gradient which is periodically amplitude-modulated along at least one spatial direction (Oppelt. A. DE 198 43 463 A1). Alternation of the gradient along at least one spatial direction reduces the local field change dB/dt which is relevant for neuronal stimulation and moreover due to the opposing forces acting in alternating gradient fields, the mechanical force and thereby the noise are at least partially reduced. One practical problem with this concept is, however, that alternation of a gradient along one direction can be realized only with magnetic field coils having a very complex construction, such that practical realization of such a gradient system has been possible only for a short time (Dennis L. Parker, J. Rock Hadley, Gradient Arrays for High Performance Multiple Region MRI. Proc. 14th Meeting ISMRM, Seattle, p. 521 (2006)). The generation of a gradient field which is unidirectional within the target volume used for the image, requires a very complex current profile, when realized with a system of a final size which is dimensioned for installation in an MR magnet, due to the mathematical structure of the Biot-Savart equations.